ci: changelog gate times out on a full-history checkout #3520

Problem

"Require changelog or skip label" (changelog-unified.yml) fails at random by timing out. Its timeout-minutes: 5 (added in #3477) is shorter than its fetch-depth: 0 checkout of the full repository history.

Evidence, 2026-09-23:

• fix: test - load ESM sources that use import.meta.url under Jest (#3472) #3496: "Checkout PR head" ran 18:45:10–18:50:30 (5 min 20 s). The gate and validation steps took 16 s. The job was then cancelled at the 5-minute limit.
• ci: dependabot - weekly npm updates and record the merge policy #3515: checkout ran 17:36:21–17:41:04; the job was cancelled in "Validate changelog schema".

That history is used only for git diff --name-only base head. The two validation scripts use only Node built-ins, so npm ci is not needed either.

labeling-unified.yml's PR labeler job has the same pattern (5-minute timeout, fetch-depth: 0), although actions/labeler reads changed files from the API and only needs its config file.

Change

• Changelog gate: list changed files with the PR files API (pulls.listFiles), not git diff. Check out only the four files the job reads (sparse, depth 1), and drop npm ci.
• Labeler: sparse, depth-1 checkout of the labeler config only.